The present invention relates to a method of operating an internal combustion engine including a plurality of cylinders as well as a control device for controlling operation of an engine in which each of a plurality of cylinders includes a plurality of valves, and at least one valve of the plurality of valves of each of the plurality of cylinders includes a fully variable valve operating mechanism.
In modern automotive engineering, OTTO engines having direct injection are used to an ever increasing extent. In direct a fuel injection, the fuel to be burned is injected directly through an injector into the combustion chamber of a cylinder.
FIG. 5 is a diagram illustrating various operating states which are possible in an OTTO engine having direct injection. The rotational speed of the OTTO engine is plotted on the abscissa of FIG. 5. Torque M of the engine is plotted on the ordinate of the diagram in FIG. 5. Reference number 1 denotes a graph delimiting a first operating state of the OTTO engine. Within the range delimited by graph 1 running from torque M1 to rotational speed N1, the engine is operated in a stratified charge operation having exhaust gas recirculation (EGR). In stratified charge operation having exhaust gas recirculation, a leaner mixture in which xcex greater than 1 is sent to the cylinders of the engine, where xcex is defined as the amount of air supplied divided by the theoretical air requirement. The lean mixture contains more air in comparison with the stoichiometric air-fuel mixture in which xcex=1.
The air-fuel ratio or mixture determines to a significant extent the torque, smooth running, fuel consumption and exhaust emissions of an OTTO engine.
In an OTTO engine including fully variable valve operating mechanism, such as an electrohydraulic valve operating mechanism (EHVS), filling of the cylinders with gas is adjusted through the valve control times of the intake and exhaust valves of the OTTO engine. In fully variable valve operating mechanism, the engine valve operations and the quantities of intake air are flexible. The quantities of intake air and residual gas for combustion in each cylinder may be controlled by varying the point in time of opening and/or closing the intake valves. This eliminates the need for air throttling of the intake air or an EGR system for adding recirculated exhaust to the intake air. In fully variable valve operating mechanism, an individual gas filling may be set for each cylinder by individual valve point in time information, i.e., through individual adjustment of the point in time of opening and/or closing of the intake valves and/or exhaust valves.
Exhaust gas recirculation is implemented in stratified charge operation in that an exhaust valve closing point in time (AS point in time), i.e., the point in time at which the exhaust valve of the cylinder is closed, is delayed to establish a valve overlap between the open states of the intake and exhaust valves within a cylinder. By controlling the amount of residual gas in the cylinder, it is possible to control nitrogen oxide emission by the OTTO engine.
In other words, the intake valve and the exhaust valve of the cylinder are controlled so that valve elevation curves of the intake valve and the exhaust valve overlap, so that residual gas from combustion remains in the combustion chamber, thus resulting in determination of a lower fresh gas filling.
In the range between graph 1 and graph 2, which runs from torque M2 to rotational speed n2, the engine is operated in homogeneous operation with exhaust gas recirculation and where lambda greater than 1 which is denoted as lean homogeneous operation.
In a range above graph 2 and below graph 3 which runs from torque M3 to rotational speed n3, the engine is operated in a homogeneous operating state. In this operating state, the OTTO engine is operated at a stoichiometric ratio, i.e., at xcex=1.
The lean operating state and stratified charge operation according to graphs 1 and 2 have up to 20% lower specific fuel consumption in comparison with the homogeneous operating state. Accordingly, for lower fuel consumption, it may be desirable to operate the OTTO engine in the lean operating state as much as possible.
However, there are narrow limits to lean stratified charge operating state. The maximum torque of a cylinder or of the OTTO engine in the lean operating state is only approx. 60% as high as the maximum torque in homogeneous operation at lambda=1, because further expansion of the lean operating state leads to production of a great deal of soot and to increased exhaust emissions. Consequently, with the OTTO engines conventionally used today, all the cylinders are operated in the lean stratified charge operating state up to a torque of approx. 60% of the maximum torque of the engine, and above this limit, i.e., above graph 2 in FIG. 5, all the cylinders are operated in the homogeneous operating state, which is less favorable in terms of fuel consumption.
Frequently, the homogeneous lean operating state is not reached at all. When the torque of the engine, in the case of a traditional OTTO engine having direct fuel injection of gasoline, rises above a value of approx. 60% of the maximum torque, all cylinders are switched from the lean stratified charge operating state to the homogeneous operating state at lambda=1. Since pressure in an intake manifold of an OTTO engine is unable to change suddenly, this switching causes jerking. It is possible to make a firing angle of the cylinder later in the homogeneous operating state until the excess intake manifold pressure has dissipated in the intake manifold in order to reduce this switching jerk while still including a high intake manifold filling. However, this only helps to dampen the switching jerk slightly. There are also disadvantages to a late firing angle in terms of fuel consumption when switching from the lean operating state to the homogeneous operating state.
It is an object of the present invention to provide a method of operating an internal combustion engine and a control device for controlling operation of an engine to permit low fuel consumption by the engine.
The method according to the present invention of operating an engine including a plurality of cylinders determines whether an engine torque is above a torque threshold value. If the specific torque is less than the torque threshold value, then all the cylinders of the plurality of cylinders of the engine are operated in the lean, stratified charge operating state. If the torque determined is greater than or equal to the torque threshold value, some of the plurality of cylinders of the engine being operated in a homogeneous operating state at lambda=1, the remaining cylinders of the plurality of engines are operated in the lean, stratified charge operating state.
This means that at an engine torque above the torque threshold value, individual cylinders are operated in the lean operating state. Thus, a ratio of a torque contribution of the cylinders in the lean operating state to a maximum torque contribution of this cylinder is equal to or less than a ratio of the torque threshold value to the maximum engine torque. Accordingly, a ratio of the respective torque contributions of the cylinders operated in the homogeneous operating state to the maximum torque contribution of this cylinder is greater than the ratio of the torque threshold value to the maximum engine torque.
The present invention may permit lower fuel consumption because the fuel consumption advantages of the lean operating state or stratified charge operation are shifted toward higher engine torques without any greater production of soot or gas emissions.
The present invention may be arranged so that the number of cylinders operated above the torque threshold value in the homogeneous operating state increases with an increase in torque. This means that progressively more cylinders are switched to the homogeneous operating state the greater the increase in engine torque above the torque threshold value.
According to this example embodiment of the present invention, not all cylinders are switched simultaneously from the lean operating state to the homogeneous operating state, but instead there is progressive incremental switching, so the present invention may permit a reduction in the switching pressure at the rotational speed threshold. Furthermore, the present invention may make it unnecessary to shift the firing angle to a later point in time in the homogeneous operating state, so this may yield an engine having reduced fuel consumption.
The control device according to the present invention for controlling operation of an engine in a plurality of cylinders, includes a plurality of valves, and at least one valve of the plurality of valves of each of the plurality of cylinders includes a fully variable valve operating mechanism, includes a comparator device for determining whether an engine torque is equal to or greater than a torque threshold value, and a valve opening control device. The valve control device is configured to control the actuation of at least one of the plurality of valves of each of the plurality of cylinders when the torque is less than the torque threshold value, such that all the cylinders of the engine are operated in lean operation. The valve-opening control device is also configured to control the actuation of at least one of a plurality of valves of the cylinder of the engine when the torque is equal to or greater than the torque threshold value, such that some of the cylinders of the engine are operated in a homogeneous operating state, the remaining cylinders of the engine being operated in the lean operating state.
The control device according to the present invention may permit fuel-saving operation of the engine. Furthermore, the control device according to the present invention has a simple configuration which may be inexpensive to produce.
The torque threshold value may be the torque at which soot production in the cylinders of the engine in the lean operating state goes beyond the required soot emission levels to be maintained. Required soot emission levels to be maintained may be, for example, maximum emission levels for soot that are established by the state.
The present invention may be arranged such that the valve opening control device increases the number of cylinders operated above the torque threshold value in homogeneous operation with an increase in torque.
This example embodiment of the present invention may permit a fuel-efficient and almost jerk-free operation of the engine when switching from a lean operating state to a homogeneous operating state.
Example embodiments of the present invention are illustrated in the accompanying figures and described below on the basis of example embodiments.